The invention relates in general to anemometers, and, in particular, to sonic (acoustic) anemometers.
There exist acoustic anemometers, also called sonic anemometers, that measure the temperature and velocity vector components of the wind in the lower atmosphere. In all cases, the anemometers are relatively large, and data is acquired at low rates. The fundamental principle behind the operation of these existing acoustic anemometers is based on the transit time for an acoustic signal to travel along a fixed path from an acoustic transmitter to a receiver.
FIG. 1A shows a prior art anemometer 10. FIG. 1B illustrates the effect of wind velocity vector U on the sound ray vectors for the single axis, dual path configuration shown in FIG. 1A. If t1 and t2 are the transit times for an acoustic signal to leave transmitters 12 and 14 and arrive at the receivers 16 and 18,
t1=d/(m cos xcex1xe2x88x92U"xgr"),
t2=d/(m cos xcex1+U"xgr"),
where U"xgr" is the wind vector component parallel to the fixed path 20, d is the path length, m is the local speed of sound in air, and xcex1=sinxe2x88x921(Uxcfx86/m) is the angle of projection of m onto Uxcfx86, the wind vector component normal to U"xgr".
Two methods can be used to at least determine U"xgr". If the transmitters produce a simultaneous acoustic signal, then the difference between the transit times can be used,       U    ξ    ≈                              m          2                ⁢                  (                                    t              2                        -                          t              1                                )                            2        ⁢        d              .  
This reduction is an approximation since it is assumed that U"xgr" less than  less than m. A second method of reduction uses the transit time reciprocal, such that       U    ξ    ≈            d      2        ⁢                  (                              1                          t              1                                -                      1                          t              2                                      )            .      
The second method is exact, and is not dependent on m. Therefore, the local ambient temperature need not be determined.
The local ambient temperature can be determined from the transit times and the normal component of the velocity vector,                                           m            2                    =                                                                      d                  2                                4                            ⁢                                                (                                                            1                                              t                        1                                                              +                                          1                                              t                        2                                                                              )                                2                                      +                          U              ϕ              2                                      ,                                          =                      γ            ⁢                          xe2x80x83                        ⁢            RT                          ,            
were xcex3 is the ratio of specific heats for air, typically xcex3=1.4, R is the gas constant for air, and T is the local ambient absolute temperature.
This method of reducing transit times into wind vector components limits the efforts in miniaturizing the measuring apparatus. A trade-off between transit time difference measurement and data acquisition rates is present. The distance d must be sufficiently long to allow high resolution of the transit time differences. Conversely, increasing the length of d requires a decrease in the tone frequency from the acoustic transmitters 12, 14, thereby limiting the highest sample rate possible with a digital data acquisition system.
Commercially available acoustic anemometers usually have d ranging between 10 to 20 cm, and the tone frequency from the transmitters at about 20 kHz. The configuration of several acoustic anemometers established in an array can simultaneously determine the three vector components of the wind, as well as the local ambient temperature. With typical dimensions, the smallest apparatus will have a sensing volume diameter of about 40 cm.
The present invention is an acoustic anemometer capable of instantly measuring all three components of the local wind velocity vector. In one embodiment the local temperature is also measured acoustically. The purpose of the anemometer is to measure the local wind velocity vector components and temperature to sufficiently high resolution such that all velocity and temperature turbulence scales are captured.